1. Field of the Invention
The present invention relates to a two-element fθ lens with short focal distance for a laser scanning unit, and more particularly to a two-element fθ lens used for a laser scanning unit with a polygon mirror, which has a short focal distance to reduce the volume size of the laser scanning unit.
2. Description of the Related Art
At present, a laser scanning unit (LSU) used in a laser beam printer (LBP) controls a laser beam scanning by a high-speed polygon mirror as disclosed in U.S. Pat. Nos. 7,079,171, 6,377,293 and 6,295,116 or TW Pat. No. 1198966, and the principles of their operation are described below: a semiconductor laser emits a laser beam through a collimator and an aperture to form parallel beams. After the parallel beams pass through a cylindrical lens, the beams are focused at the width of the Y-axis in the sub scanning direction and along a direction parallel to the X-axis of the main scanning direction to form a line image and projected onto a high-speed polygon mirror. The polygon mirror includes a plurality of continuous reflecting mirrors disposed precisely at or proximate to the focal point of the line image. The polygon mirror is provided for controlling the direction of projecting the laser beam, so that when a plurality of continuous reflecting mirrors are rotated at a high speed, the laser beam projected onto a reflecting mirror can be extended in a direction parallel to the main scanning direction (x-axis) at the same angular velocity and deviated from and reflected onto a fθ lens. The fθ lens is installed next to the polygon mirror and can be either a single-element lens structure (or a single-element scanning lens) or a two-element lens structure. The function of this fθ lens is to focus a laser beam reflected by the reflecting mirror of the polygon mirror and projected onto the fθ lens into a circular or oval spot that is projected onto a photoreceptor surface (or a photoreceptor drum, which is an imaging surface) to achieve the requirements of the scanning linearity as disclosed in U.S. Pat. Nos. 4,707,085 and 6,757,088 and Japan Pat. No. 2004-294713. However, the traditional fθ lens still has the following drawbacks.
(1) When a laser beam is reflected from a polygon mirror, the central of the laser beam projected onto a reflecting mirror of the polygon mirror is not aligned physically with the central rotating axis of the polygon mirror, then the design of a fθ lens has to take the issue of a reflection deviation of the polygon mirror into consideration. In a traditional optical correction method, a sub scanning direction is used for correcting the optics of a main scanning direction as disclosed in U.S. Pat. Nos. 5,111,219 and 5,136,418 and Japan Pat. No. 2756125. A longer focal length is required to correct the reflection deviation by the sub scanning direction appropriately, but such arrangement also increases the volume of the laser scanning unit.
(2) To meet the specifications and using requirements for the diameter of the spot formed on the drum by the scanning light of the fθ lens, the prior art generally adopts a longer focal length to provide a better image quality, or even uses a reflecting mirror to extend the imaging distance as disclosed in U.S. Pat. No. 2002/0063939; or a three-element lens as disclosed in U.S. Pat. Nos. 2002/0030158 and 5,086,350, and Japan Pat. No. JP63-172217; or a hard-to-manufacture diffraction lens as disclosed in U.S. Pat. Nos. 2001/0009470 and 5,838,480; or a two-element lens having an inflection point as disclosed in U.S. Pat. Nos. 5,111,219, 7,057,781 and 6,919,993; or a single-element lens having an inflection point as disclosed in Japan Pat. No. JP04-50908.
(3) For the applications of a small printer, the imaging distance of the drum is decreased to reduce the volume of a laser scanning unit (LSU) as disclosed in U.S. Pat. No. 7,130,096 and adopts a method of limiting the ratio of the effective scanning range and the image optical distance (optical length) to reduce the imaging distance on the drum and eliminate ghost images; U.S. Pat. No. 6,324,015 adopts a method of limiting the distance (or focal distance) between the polygon mirror and the drum and the focal length ratio (d/f) of the fθ lens to reduce the distance, wherein the focal distance is approximately equal to 200 mm, and the focal length is equal to 100 mm for the illustration; U.S. Pat. No. 6,933,961 discloses a method of limiting the distance from the last spot (or the end of the scanning line) to an optical surface of the fθ lens, but the maximum scanning angle is approximately equal to 27.6 degrees, which cannot reduce the focal distance effectively.
To satisfy consumer requirements for a light, thin, short and compact design of the laser scanning unit, a two-element fθ lens with a short focal distance (such as a focal distance less than 150 mm for an A4 sized laser printer) may be adopted the needs in effective correcting optical distortion in the main scanning and sub scanning directions as well as in improving the scanning quality and the resolution.